The A-137-2 Wave Multiplier II is a very nice module which can do very subtle as well as quite extreme things. Here’s an example with an A-111 (triangle wave) and four independent LFOs (triangle waves from an A-143-3):
It begins with slight modulation of a single quite slow LFO, then the other LFOs were added. Increasing CV levels and LFO frequencies. Then I switch all LFOs to high speed, with maximum modulation depths in the A-137-II.
Technical deatails can be found at Doepfer’s homepage: